Method for controlling sensitivity of gas chromatograph



United States Patent 0 METHOD FOR CONTROLLING SENSITIVITY OF GAS CHROMATOGRAPH Jaroslaw Kaczaj, 502 Franklin Ave., Cheltenham, Pa. 19012 No Drawing. Filed Apr. 29, 1968, Ser. No. 725,159 Int. Cl. G01n 31/08, 31/12 US. Cl. 23-232 10 Claims ABSTRACT OF THE DISCLOSURE Method of restoring maximum sensitivity of a gas chromatograph equipped with a flame ionization detector which comprises injecting in to the chromatograph a minor quantity of a substance selected from the group consisting of fluorinated alcohols and esters of fluorinated alcohols and allowing said substance to pass through the chromatograph.

BACKGROUND OF THE INVENTION A gas chromatograph is an instrument which separates and analyzes the vaporized components of a chemical mixture. It has found important use both as a laboratory tool and as a means of automated control in chemical production. Basically, the chromatograph comprises a source of inert carrier gas, a means of injecting the sample to be analyzed into a stream of the carrier gas, an adsorption column packed with small substrate particles which selectively adsorb and separate the components of the sample, a detecting device capable of sensing the arrival of the separated components and a recorder fed by the output of the detecting device.

The detecting device most commonly used in the analysis of organic materials when high sensitivity is required, as for example when minute quantities of impurities are to be detected, is the flame ionization detector. At the entrance of this detector, hydrogen gas and a source of oxygen is mixed with the carrier gas and ignited to produce a continual flame. The ions produced in this flame are increased whenever a separated chemical component enters the flame. The sensing mechanism includes a potential field to move these ions and measuring circuit to convert the ion current to a recorder signal. The sensitivity of the detecting device is measurable under standardized conditions as the response in units on the recorder graph paper per known quantity of a standard pure material. Unfortunately the sensitivity does not remain constant with continued operation of the instrument and must be re-determined from time to time by calibration with appropriate standard pure substances. The rate of sensitivity decrease depends on the nature and composition of the samples introduced into the chromatograph and on the nature of the substrate particles which are frequently silicone coated, and which coatings tend to bleed off and contaminate the collector electrode with silicone material.

Another frequent source of collector electrode contamination is encountered when anti-tailing agents such as trimethylsilyl compounds or their analogs are used to improve the resolution properties of the adsorption column. Without the use of such agents the column member of the chromatographic system often becomes contaminated with small amounts of persistently adsorbed impurities which then change the respective aflinities of the column for the components to be separated in an analytical sample, resulting in the holdout of some components known as tailing and the subsequent overlapping of the arrival zones of the successive components. To re store the column to its top resolution efliciency, a well established corrective procedure is the injection of a d trimethylsilyl compounds which convert the adsorbed impurities into volatile derivatives that then pass through the system. However, since these derivatives and the trimethylsilyl compounds themselves are silicon compounds they possess the undesirable property of now contaminating the collector electrode.

There are thus several instances which eventually cause sufficient contamination of the collector electrode to make cleaning of the electrode necessary in order to restore maximum sensitivity, i.e., maximum response in units of the recorder graph paper per known quantity of a standard pure material.

It eventually becomes necessary to clean the detector in order to restore sensitivity. In prior practice the restoration of sensitivity has required cumbersome and timeconsuming mechanical steps. It has been necessary to disassemble the fragile parts of the flame ionization detector and carefully remove solids deposits of silicone material from the collector electrode which usually is of a platinum composition. Several hours are usually needed for this mechanical cleaning procedure and great delicacy must be used to remove foreign matter from the metal surfaces without abrading the metal parts themselves.

Another prior method for cleaning the detector encompasses disassembling the parts of the detector and cleaning the collector electrode with hydrofluoric acid. This method was disadvantageous because connections to the collector electrode were corroded by the hydrofluoric acid and the method itself was time consuming.

SUMMARY OF THE INVENTION We have now found a means of restoring the flame ionizationdetector of a gas chromatograph to maximum sensitivity without dismantling the same. In brief our invention comprises the procedure of injecting into the chromatograph a minor quantity of a substance selected from the group consisting of fluorinated alcohols and esters of fluorinated alcohols and allowing said substance to pass through the chromatograph. It has been surprisingly found that this procedure results in automatic cleaning of the flame ionization detector with complete restoration of the sensitivity to its maximum level.

DETAILED DESCRIPTION OF THE INVENTION When it is determined that the sensitivity of a gas chromatograph has decreased and must be restored, a material or substance selected from the class consisting of fluorinated alcohols and esters of said alcohols is injected, via a hypodermic syringe, through a self-sealing plug into the carrier gas which is used to continually purge the column of the chromatograph. The carrier gas also carries the sample through the chromatograph.

The cleaning material then passes through the column and into a flame ionization detector. The flame detector, which is fed with a mixture of hydrogen and oxygen is cleaned by the fluorinated alcohol or ester.

The exhaust of the alcohol or ester gas is then passed out of the chromatograph.

Any fluorinated alcohol having from about 2 to about 23 carbon atoms and preferably from about 5 to about 13 carbon atoms may be used. Additionally, the fluorinated alcohol may have up to about 44 fluorine atoms thereon and preferably from about 8 to about 25 fluorides.

Generally speaking, the classes of fluorinated alcohols which are useful in this invention conform to the following formulae:

CF CF 2 CH OH where 11:0 to 12;

CHF 'CF (CF CF CH OH and OHFZCF2 CF20F2).CH01I where x= to 10; R is H, CH C H butyl, hexyl, and the like and CF3-CHOH The esters of these alcohols which are useful in this invention are the acetates, propionates, butyrates, phosphates, sulfates and the like.

Among the alcohols and esters which may be used are monofluoropropyl alcohol, 1,1-dihydroheptafiuorobutly alcohol, 2,2,2 trifluoroethyl alcohol, 1,1,7 trihydroperfluoro heytpl alcohol, trihydroperfluorononyl alcohol, 1,1,7 trihydroperfluoro heptyl acrylate, trihydroperfluoro propyl acetate and the like.

It is preferred that the boiling point of the ester or alcohol used be between about 50 C. and about 130 C. for ease of operability.

The fluorinated alcohols and esters are used as cleaning agents in the process of this invention in an amount sufficient to clean the flame ionization detector. Generally, a total dosage of about 5 microliters is more than suflicient for this purpose so that a single injection will usually be satisfactory. However, it is preferred to inject successively several smaller doses of precisely measured cleaning agent, for example 1.00 microliter. This procedure advantageously enables one to determine when the system has reached maximum sensitivity by observing when two successive doses of equal magnitude result in recorder responses of equal magnitude.

The operator will know when the chromatograph has been cleaned and sensitivity restored to the flame detector by the recorder response to the sample or to a standard having a predetermined maximum response.

If the maximum response is not received after the cleaning agent is injected into the carrier gas, then the procedure is repeated until maximum response is received.

The maximum response received may be that of the fluorinated alcohol or ester, it may be the maximum recorder response of a known standard such as dibutyl fumarate.

The fluorinated substances of this invention may be used as combination calibration and cleaning agent in a manner completely independent of any prior fixed notion-s as to preferred operating conditions. In research it is often desirable to explore various experimental conditions of the chromatographic system in working toward optimum conditions for wide separation of the components of mixtures. In such cases there is no a priori knowledge of the setting for maximum sensitivity with regard to any particular substance since entirely new combinations of variables are being used.

In order to more fully illustrate the nature of this invention and the manner of practicing the same, the following examples are presented.

EXAMPLE 1 Several series of samples containing mixtures of organic substances were analyzed in sequence on an 810 Model Research Chromatograph, manufactured by F and M Scientific Corporation, Avondale, Pa.

During the course of a week of almost uninterrupted use, the columns employed, which were stainless steel, included alternatively a four foot column packed with mesh diatomaceous earth coated with 20% by Weight of silicone rubber, a ten foot column of diatomaceous earth coated with 15% by weight of Dow Corning 200 silicone grease and a 10 foot column of 60 mesh firebrick coated with 15% by weight of diethylene glycol succinate.

Helium was used as the carrier gas. 1 microliter size samples were introduced through the usual syringe -type glass injector which was of 10 microliter size. The chromatograph was equipped with a flame ionization detector having a platinum electrode system. The signal from the detector was fed to a Minneapolis-Honeywell recorder of 0.2 to 1.0 millivolt range.

At the beginning of said weeks work some quantitative analyses were run based on standardization with dibutyl fumarate, using the silicone rubber coated column maintained at a temperature of 210 C. Standard conditions for the calibration included a helium flow rate of 60 milliliters per minute, detector temperature at 300 C., injection port temperature at 230 C. and an appropriate attenuation setting of the recorder. Under these conditions it was observed that 1.00 microliter of the standard substance, dibutyl fumarate, gave a response of 99 units on the recorder graph paper.

At the end of the weeks analyses, which included for the most part qualitative analyses and did not require repeated calibrations, it was observed that the sensitivity had deteriorated to the degree that variations due to noise were interfering with a correct interpretation as to what responses corresponded to the appearance of separated components. When the equipment was recalibrated with the same standard dibutyl fumarate and the appropriate aforementioned column under the identical. standard conditions of helium flow rate, attenuation setting, etc., it was now found that the sensitivity had indeed quantitatively deteriorated to only 50 recorder chart units.

At this point 5 microliters of C F alcohol, namely HCF CF CF CF CFQCF CH OH, (trihydroperfluoroheptyl alcohol) was introduced into the injection port, with the same standard conditions of helium flow, operating temperatures, etc., as above-mentioned. The several peaks corresponding to substances comprised in the alcohol sample were recorded in a time interval of between 10 and 15 minutes. At this point 1.00 microliter of standard substance dibutyl fumarate, was again run through the equipment, and found to have a response in the recorder of 100 units, demonstrating that the cause of low sensitivity had been completely removed from the apparatus and that the chromatograph had been restored to full sensitivity.

EXAMPLE 2 It is also possible to use the fluorinated alcohol or ester of this invention directly as the calibrating material. The fluorinated alcohol used in Example 1 is run through a chromatograph prior to using said chromatograph for a quantitative analysis of an unknown sample. The fluorinated alcohol gives the recorder response of 100 units which has been found by experience to be the maximum response for the particular set of conditions used in the chromatographic analysis. This demonstrates that the chromatograph is ready for quantitative analysis of a sample at maximum sensitivity setting.

On the other hand, if the response of the fluorinated alcohol had been less than the number of units on the recorder chart known to correspond to maximum sensitivity (100 units), repeated samples of the fluorinated alcohol are passed through the chromatograph until the recorder response confirms that maximum sensitivity has been established.

EXAMPLE 3 The chromatograph of Example 1 is fitted with a proprietary column containing a proprietary adsorbent and tested experimentally with an injection of 1.00 microliter of the fluorinated alcohol of Example 1 under the standard conditions specified in Example 1 except that the attenuation setting is arbitrary. The recorder response is 56 units. A second 1.00 microliter sample of the fluorinated alcohol is injected under the same conditions and attenuation setting. The response is 77 units. A third 1.00 microliter injection of the fluorinated alcohol under the same conditions and attenuation setting gives a response of 76 units. The last two successive samples having given identical recorder responses within experimental error, the chromatograph is demonstrated to be both cleaned and calibrated for further use. It is a matter of choice whether the attenuation setting should be left to give a standard response of 76-77 units or whether it should be changed for convenience to read 100 units when 1.00 microliter of the fluorinated alcohol are passed through the chromatograph.

EXAMPLE 4 The chromatograph of Example 1 was used for an extended period of time with a column packed with diethylene glycol succinate coated on firebrick. During this time, operation at maximum sensitivity was maintained by the method of Example 2. However, eventually the apparatus became inefiicient in regard to degree of resolution. A standard mixture of two organic compounds which had previously been well separated on this column now were poorly resolved and the arrival of the second component was spread over a wide period of time exhibiting the phenomenon known as tailing. This indicated that the adsorption column might have been contaminated with residues that were interfering with its optimum selective adsorption characteristics. To counteract this, 12 microliters of a trimethyl silyl conditioner was injected into the column at 200 0, resulting in a plurality of peaks on the recorder indicating that the column had indeed been contaminated.

It was now found, however, that the steps taken to correct the resolving efficiency of the column had impaired the sensitivity. When 1.00 microliter of fluorinated alcohol of Example 1 was injected into the chromatograph, the recorder response was only 81 units instead of 100 units expected for maximum response under the conditions used. This indicated that some of the silicaceous system used for column-correction had contaminated the detector.

Two more 1.00 microliter injections of the fluorinated alcohol of Example 1 were successively made. This restored sensitivity to its maximum level as indicated by a recorder response of 100 units.

EXAMPLE 5 The procedure of Example 1 is repeated several times except that the cleaning agent used is changed each time. The cleaning agents used are monofluoropropyl alcohol, 2,2,2 trifiuoroethyl alcohol, 1,1,2,2,3 pentafluoropropyl alcohol, 1,1,11 trihydroperfluoroundecanol, 1,1,22 trihydroperfluoro docosanol, 1,1,7 trihydroperfluoroheptyl acrylate, the acetate ester of 1,1,22 trihydroperfluoro docosanol, 1,1,7 trihydroperfluoroheptyl butyrate, and monofluoropropyl sulphate. In each case good results are obtained.

' While this invention has been described in terms of certain preferred embodiments and illustrated by means of specific examples, the invention is not to be con strued as limited except as set forth in the following claims.

I claim:

1. A process for cleaning a gas chromatograph equipped with a flame ionization detector comprising the steps of maintaining said chromatograph in an intact assembled operative condition, introducing into said chromatograph a material selected from the class consisting of fluorinated alcohols and esters of said alcohols, passing said material through said chromatograph, contacting said flame ionization detector with said material and passing said material out of said chromatograph.

2. A process according to claim 1, wherein said alcohol has up to 23 carbon atoms.

3. A process according to claim 1, wherein said alcohol is an aliphatic alcohol.

4. A process according to claim 1, wherein said alcohol has up to 44 fluorine atoms.

5. A process according to claim 1, wherein said fluorinated alcohol is selected from the class consisting of monofluoro propyl alcohol, 1,1, dihydroheptafluorobutyl alcohol, 2,2,2 trifiuoroethyl alcohol, and 1,1,7 trihydroperfluoroheptyl alcohol.

6. A process according to claim 1, wherein said ester is selected from the class consisting of trihydroperfluoro propyl acetate and 1,1,7 trihydroperfluoroheptyl acrylate.

7. A process according to claim 1, wherein said alcohol is trihydroperfluoroheptyl alcohol.

3. A process according to claim 1, wherein said alcohol is trihydroperfluorononyl alcohol.

9. A process according to claim 1, wherein said ester is 1,1,7 trihydroperfluoroheptyl acrylate.

10. The process of cleaning a gas chromatograph equipped with a flame ionization detector comprising the steps of maintaining said chromatograph in an intact assembled operative condition, introducing into said chromatograph a material selected from the class conforming to the following formulae;

CF CF CH OH where 12:0 to 12;

CHF CF CF CF CH OH and CHFQC Mo FgoFnxtllHoH where x=0 to 10 and R is selected from the group consisting of H, CH C H butyl, hexyl and CF -CHOH and esters of said alcohols selected from the class consisting of acetates, propionates, butyrates, phosphates and sulphates, passing said material through said chromatograph, and contacting said flame ionization detector with said material.

References Cited Phillips et al., Gas Chromatography, 1960, London, Butterworths. Edited by R. P. W. Scott, p. 310.

Felton, H. R., Gas Chromatography, Third International Symposium, June 13-16, 1961, Academic Press. Edited by Brenner et al., pp. 437-441.

MORRIS O. WOLK, Primary Examiner R. M. REESE, Assistant Examiner 233 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,531,256 Dated September 9, 970

Inventor) Jaroslaw Kaczaj It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 4, after "Cheltenham, Pa. 19012" please insert --Assignor to Borden, Inc., New York, N.Y.,, a corporation of New Jersey-- Column 3, line 28, "butly" should be --butyl-- Column 3, line 29, "heytpl" should be --hepty1- $IGN'ED PND SBILILD FEB 2 131971 tSEAL) Am mm 1:. SQHUYLER, .m MOM Gomissioner of Patents 

